1. Field of the Invention
The present invention relates to an oil-well pipe joint with improved sealing properties and galling resistance and to a method for the manufacture thereof, and particularly to an oil-well pipe joint which comprises a blast-plated layer of soft metal particles at least on the surface of a metal sealing portion and which is free from delayed fracture and a manufacturing method therefor.
2. Description of Related Art
Recently, the environment in which oil-wells and gas-wells are being drilled is becoming increasingly severe. In such an environment, oil well tubes such as drill pipes and casing pipes are usually subjected to a high pressure such as 3.5 .about.14.0 kgf/mm.sup.2 (5,000.about..degree.,000 psi), so that a high level of sealing properties is required for oil well pipe joints. The same applies to gas well pipe joints. Therefore, these pipes and joints are collectively referred to as "oil well pipes" and "oil well pipe joints" respectively hereinafter in this specification.
In general, oil well pipes are connected by joints and must be used repeatedly. Therefore, an oil well pipe is always subjected to connecting and disconnecting. An oil well pipe joint comprises a threaded portion and a metal sealing portion. In order for the joint to exhibit adequate sealing properties to endure the above-mentioned high pressure, a high degree of contact pressure must be applied to the metal sealing portion as well as the threaded portion.
Thus, the occurrence of galling of the metal sealing portion as well as the threaded portion is unavoidable when the connecting and disconnecting are repeated.
Therefore, in order to prohibit the occurrence of galling and to strengthen the sealing properties, a surface treatment is applied to the metal sealing portion of an oil well pipe joint. Usually the surface treatment includes the formation of a chemically-formed film, such as one of zinc phosphate, manganese phosphate, and iron oxalate. However, these surface films deteriorate when the service temperature goes over 200 C., resulting in "leakage".
Currently the environment of oil-wells is getting more severe, and it is not usual for oil well pipe joints to be used at a temperature of 300.degree. C. or more, e.g. 300.degree. C..about.350.degree. C. Thus, it is said that the provision of a plating layer of a soft metal having a high melting point is effective to cope with such deterioration of the chemically-formed film.
In addition, since the environment of the oil wells is getting more severe, a high-strength high-alloy material which is not described in API specifications is increasingly employed.
API specifications require a strength on the order of 55.about.110 ksi (1 ksi=0.7 kgf/mm.sup.2) and carbon steels as materials. However, the strength level required at present is a yield strength of 140.about.155 ksi and the type of the high-alloy now employed is that containing 5.about.45%, and usually 9.about.25% of Cr and 0.about.60% of Ni. Only electroplating is applicable to such high alloy materials, but it is not easy to carry out electroplating since it is rather difficult to activate the surface of the alloy, resulting in inadequate resistance to galling.
U.S. Pat. No. 4,474,651, for example, proposes the provision of an activated layer on the metal sealing portion or threaded portion of a pipe joint before metal plating.
However, even if such an activated layer is provided, the pin itself is as long as 9.about.12 m, and there are many difficulties in carrying out metal plating of such a long member so far as a plating apparatus therefor and operating conditions are concerned. Therefore, such a method can not be immediately employed due to problems with its practicality.
In addition, when an electroplating is applied to a high strength material, a delayed fracture occurs due to the presence of hydrogen penetrated from a plating bath into an electroplated layer and the base steel. Should such a fracture occur, needless to say, there is the fear of a serious accident taking place in the oil well. Complete precautions are necessary to avoid such troubles.
As oil well pipe joints employ increasingly stronger and higher-alloy materials, degradation in sealing properties and galling resistance has been experienced. An improvement in these properties is highly desired.
In order to avoid the before-mentioned delay fracture, a heating treatment called "baking" is carried out after electroplating to diffuse hydrogen. However, such a treatment has the drawbacks that it takes a long time and it can not suppress the delay fracture. An improvement in the resistance to delay fracture is also desired.
One known dry-plating method is blast plating. As for the materials for use in blasting, Japanese Patent Publication No. 9312/1984 discloses a blasting material comprising a core of iron or an iron alloy covered with zinc or a zinc alloy coating with a layer of a zinc-iron alloy therebetween. Blasting plating is carried out as a dry plating method so as to improve the corrosion resistance of a steel substrate. However, the corrosion resistance of the resulting plating is not enough for such purposes. Therefore, it is used for only very limited applications. Furthermore, so far as the blasting material disclosed in the above-mentioned Japanese patent is concerned, an improvement in the corrosion resistance of the resulting plating is not expected, and an additional surface treatment is required. Thus, the blasting material has not yet been satisfactorily utilized.